The present invention generally relates to an optical disk substrate of an optical recording medium for effecting an information recording, reproducing or erasing operation using an optical means such as a laser beam or the like.
The construction of a conventional optical disk is basically shown in FIGS. 16(a) and 16(b). A recording film is provided through a protective layer on a disk substrate, and further, a protective film is provided on the recording film through an intermediate dielectric film.
In order to effect recording and erasing operations in a magneto-optical recording medium using a perpendicular magnetization film having a magneto-optical effect in the recording film, the recording film is locally heated to temperatures small in coercive force of compensation temperatures or to approximately the curie temperature or higher by the application of laser beams so as to lower the coercive force of the recording film in the application portion for enabling the magnetization of the recording film in the direction of the external magnetic field (so called thermomagnetic recording). In order to effect the reproduction of the recording signal, laser beams which have a lower intensity than the laser beams used during recording or erasing are used so as to detect photons with an analyzer, as change in optical intensity, a situation where the polarization plane of the reflected light or transmitted light is rotating (which is caused in accordance with the magneto-optical effect of the so called Kerr effect and Faraday effect) in accordance with the recording condition of the recording film, namely, the magnetization direction. A magnetic material having a perpendicular magnetic anisotropy is used for the recording film of the magneto-optical recording medium in order to effect a high density recording operation with interference among the inversely directed magnetization being made smaller.
Optical beams are applied on the medium likewise, in the case of an optical disk using a phase-change (pc) optical recording medium, having an organic material for the recording film, so as to cause a local temperature rise or chemical change because of optical absorption for effecting the recording operation. The signal is reproduced by the application of optical beams of different intensity or wavelength from those used during recording in the local change in the medium caused by the recording operation so as to detect the reflected light or the transmitted light.
In the conventional optical disk, an optical disk substrate provided with guide grooves for the tracking guide of the optical spots or prepits is used so that high density recording and random access are realized.
In the molding operation of the actual optical disk substrate, the intervals between the tracking grooves or prepits are close so that the transferring operation from the stamper to the disk substrate is insufficient. FIG. 16(b) is a view showing the appearance of the resin material of the conventional stamper and disk substrate during manufacture. FIGS. 17(a) and 17(b) are views showing the relationship between the prepits of the stamper and the land planes. The land planes to be described hereinafter are regions shown with oblique lines adjacent to the prepits 53, of the faces disposed between the prepits 53 and the grooves 52 as shown in FIG. 17(a), or of the faces disposed with tracks formed by the prepits 72 as shown in the stamper of the optical disk of the ROM type of FIG. 17(b).
In such a stamper as shown in FIG. 16(b), the flowing condition of the molten resin is shown by arrows when the plastic resin material molten such as polycarbonate or the like is injected from the substrate center A side, because the depth of the prepits 83 is set to be approximately x/4n with respect to the laser wavelength x during the reading time and the refractive index n of the substrate. Although the resin material is likely to flow to the land 88 where the prepits of stamper on the internal peripheral side are not formed with respect to the prepits 83 from the relationship between the viscosity of the resin material and the track pitch, the resin material is flowed sufficiently no longer to the land 89 on the external peripheral side.
Also, the difference in depth between the land 84 on the internal peripheral side and the land 85 on the external peripheral side of the substrate molded with respect to the prepits 83 of the disk substrate 81 becomes larger when the track pitch becomes smaller. Especially the transferring operation of the prepits and the adjacent land planes become difficult to effect when the track pitch is made smaller for high recording density. Errors are caused, because the discrimination of the sectors and the addresses cannot be effected when the servo characteristics and the recording and reproducing characteristics during the recording and reproducing time are lowered or the transferring operation of the prepits is insufficient. When the transferring operation is forced to effect on the land plane on the external peripheral side with respect to the prepits with the compressing pressure at the injection time being made larger, the tilt or the eccentric acceleration becomes larger, thus causing a problem in the mechanical characteristics.